As vehicles such as automobiles become high-end, it is becoming common to control the deployment of an air bag for protecting an occupant when in a car crash, etc., based on the posture of the occupant and so forth. As such a technique for sensing a posture of an occupant, the head position detecting system disclosed in Patent Document 1 identified below is known, for example.
This head position detecting system includes: an oscillating unit which includes an oscillating electrode provided in a seat portion (seating portion) and capable of becoming electrically conductive with an occupant sitting (seated) on the seat (seating) through a high-frequency wave and which applies an oscillating output between the oscillating electrode and a metal frame; and a distance measuring sensor including a receiving electrode which is provided in an insulated state in the ceiling of the vehicle interior to be opposite to the oscillating electrode.
This head position detecting system further includes: a calibration sensor which performs measurement with a calibration receiving electrode which is provided in an insulated state, at a known distance from a conductive surface which becomes electrically conductive with an occupant, and to be opposite to that conductive surface; and a processing circuit which determines characteristic curves between distances and output voltages of the receiving electrode based on the result of the measurement, and fits the value of each output voltage to a corresponding characteristic curve to obtain a distance. The head position detecting system detects a position of a head by measuring a distance between the head of the occupant and the receiving electrode.